Pete Garrison is a Secret Service agent and one of the personal bodyguards for First Lady of the United States Sarah Ballentine, with whom he is having an affair.

The Secret Service Protective Intelligence Division, led by Garrison's estranged friend and former protégé, David Breckinridge, with rookie partner Jill Marin, is tasked with investigating the plot.

The agent in charge of the Presidential Protective Division, William Montrose, decides to randomly select the means of transporting the president using a coin toss.

As the President and first lady visit Camp David, Garrison's informant calls, demanding that his payment be made at a shopping mall food court.

Simultaneously, the presidential helicopter is shot down by a surface-to-air missile outside of Camp David, though neither the President nor his wife was aboard (owing to Montrose's coin "deciding" to use the motorcade instead).

Together in Toronto, Garrison and Breckinridge learn that the assassins are former KGB operatives hired to kill the president by a Colombian cartel and the mole, William Montrose, who was never polygraphed.

Emotionally torn, Montrose is instructed to jam Secret Service's radios, and leave the summit with the President via a specific route; the assassins will handle the rest.

The assassins, posing as Royal Canadian Mounted Police Emergency Response Team officers, kill several agents and corner Montrose and the President in a tunnel.

The site's consensus reads, "The Sentinel starts off well enough but quickly wears thin with too many plot holes and conventional action sequences.